I would like to impose periodic BC to my linear elastic system (2D). I need the *derivative* of one component of the displacement field to be equal on respective edges, i.e. {u_y},x on left = {u_y},x on right. ,x denotes partial derivative along x and u_y is the y-component of the displacement vector u.
I know that there is no brick responsible for this in GetFEM, so I kindly ask for some help. I would expect it to be done with the use of additional system of Lagrange multipliers. Is that correct?